The Base Excision Repair system of Salmonella enterica serovar typhimurium counteracts DNA damage by host nitric oxide

Inducible NO synthase and antibacterial host defence in times of Th17/Th22/T22 immunity

During the last two decades nitric oxide (NO) produced by inducible NO synthase (iNOS or NOS2) has been characterized as immunoregulatory and antimicrobial principle displaying the potential to determine course of disease in a range of infections. Being an enzyme primarily regulated on expressional level, cytokine-driven iNOS appears to be connected in particular with activation of Th1-type immunity. However, with the recent advent of additional, partly overlapping CD4(+) T cell effector subsets, namely Th17 and Th22 cells, a further layer of complexity has been added to immunoregulatory networks determining inflammatory gene expression in the context of microbial infections. Here, we review current knowledge on activation of iNOS function by interleukin (IL)-17 and IL-22 with focus on Th17/Th22-directed antibacterial immunity.

DNA damage and reactive nitrogen species are barriers to Vibrio cholerae colonization of the infant mouse intestine

Ingested Vibrio cholerae pass through the stomach and colonize the small intestines of its host. Here, we show that V. cholerae requires at least two types of DNA repair systems to efficiently compete for colonization of the infant mouse intestine. These results show that V. cholerae experiences increased DNA damage in the murine gastrointestinal tract. Agreeing with this, we show that passage through the murine gut increases the mutation frequency of V. cholerae compared to liquid culture passage. Our genetic analysis identifies known and novel defense enzymes required for detoxifying reactive nitrogen species (but not reactive oxygen species) that are also required for V. cholerae to efficiently colonize the infant mouse intestine, pointing to reactive nitrogen species as the potential cause of DNA damage. We demonstrate that potential reactive nitrogen species deleterious for V. cholerae are not generated by host inducible nitric oxide synthase (iNOS) activity and instead may be derived from acidified nitrite in the stomach. Agreeing with this hypothesis, we show that strains deficient in DNA repair or reactive nitrogen species defense that are defective in intestinal colonization have decreased growth or increased mutation frequency in acidified nitrite containing media. Moreover, we demonstrate that neutralizing stomach acid rescues the colonization defect of the DNA repair and reactive nitrogen species defense defective mutants suggesting a common defense pathway for these mutants.

Studies on intracellular bacterial pathogens have shown the need for maintaining genomic fidelity to promote colonization. Loss of DNA repair functions often leads to attenuation and rapid clearing of the invading pathogen. However, for some pathogens, an increased mutation rate has been shown to be beneficial for promoting host colonization, presumably by allowing the pathogen to rapidly adapt to adverse host conditions. We asked if the non-invasive pathogen V. cholerae experienced increased DNA damage during infection and if so, how the increased damage influenced host colonization and from where the source of the damage was derived. Our results demonstrate that V. cholerae experiences increased DNA damage during infection in the infant mouse model and that loss of ability to repair this damage results in attenuation of virulence. We specifically show that V. cholerae requires both base excision repair and mismatch repair for efficient intestinal colonization. Furthermore, we present evidence that the source of the DNA damage is derived from reactive nitrogen species (RNS) formed by acidified nitrite in the mouse gut and in doing so we identify a new RNS defense protein found in V. cholerae and several other pathogenic bacteria.

Identification of a chemical that inhibits the mycobacterial UvrABC complex in nucleotide excision repair

Bacterial DNA can be damaged by reactive nitrogen and oxygen intermediates (RNI and ROI) generated by host immunity, as well as by antibiotics that trigger bacterial production of ROI. Thus a pathogen’s ability to repair its DNA may be important for persistent infection. A prominent role for nucleotide excision repair (NER) in disease caused by Mycobacterium tuberculosis (Mtb) was suggested by attenuation of uvrB-deficient Mtb in mice. However, it was unknown if Mtb’s Uvr proteins could execute NER. Here we report that recombinant UvrA, UvrB, and UvrC from Mtb collectively bound and cleaved plasmid DNA exposed to ultraviolet (UV) irradiation or peroxynitrite. We used the DNA incision assay to test the mechanism of action of compounds identified in a high-throughput screen for their ability to delay recovery of M. smegmatis from UV irradiation. 2-(5-Amino-1,3,4-thiadiazol-2-ylbenzo[f]chromen-3-one) (ATBC) but not several closely related compounds inhibited cleavage of damaged DNA by UvrA, UvrB, and UvrC without intercalating in DNA and impaired recovery of M. smegmatis from UV irradiation. ATBC did not affect bacterial growth in the absence of UV exposure, nor did it exacerbate the growth defect of UV-irradiated mycobacteria that lacked uvrB. Thus, ATBC appears to be a cell-penetrant, selective inhibitor of mycobacterial NER. Chemical inhibitors of NER may facilitate studies of the role of NER in prokaryotic pathobiology.

Host DNA repair proteins in response to Pseudomonas aeruginosa in lung epithelial cells and in mice

Although DNA repair proteins in bacteria are critical for pathogens' genome stability and for subverting the host defense, the role of host DNA repair proteins in response to bacterial infection is poorly defined. Here, we demonstrate, for the first time, that infection with the Gram-negative bacterium Pseudomonas aeruginosa significantly altered the expression and enzymatic activity of 8-oxoguanine DNA glycosylase (OGG1) in lung epithelial cells. Downregulation of OGG1 by a small interfering RNA strategy resulted in severe DNA damage and cell death. In addition, acetylation of OGG1 is required for host responses to bacterial genotoxicity, as mutations of OGG1 acetylation sites increased Cockayne syndrome group B (CSB) protein expression. These results also indicate that CSB may be involved in DNA repair activity during infection. Furthermore, OGG1 knockout mice exhibited increased lung injury after infection with P. aeruginosa, as demonstrated by higher myeloperoxidase activity and lipid peroxidation. Together, our studies indicate that P. aeruginosa infection induces significant DNA damage in host cells and that DNA repair proteins play a critical role in the host response to P. aeruginosa infection, serving as promising targets for the treatment of this condition and perhaps more broadly Gram-negative bacterial infections.

Visiting the cell biology of Salmonella infection

Salmonella, a Gram-negative facultative intracellular pathogen is capable of infecting vast array of hosts. The striking ability of Salmonella to overcome every hurdle encountered in the host proves that they are true survivors. In the host, Salmonella infects various cell types and needs to survive and replicate by countering the defense mechanism of the specific cell. In this review, we will summarize the recent insights into the cell biology of Salmonella infection. Here, we will focus on the findings that deal with the specific mechanism of various cell types to control Salmonella infection. Further, the survival strategies of the pathogen in response to the host immunity will also be discussed in detail. Better understanding of the mechanisms by which Salmonella evade the host defense system and establish pathogenesis will be critical in disease management.

A new approach utilizing real-time qPCR to detect in vitro base excision repair

DNA lesions in mammalian cells may be induced by reactive oxygen species, alkylation, and ionizing radiation. This damage can then be repaired via the base excision repair (BER) pathway, which includes single strand break repair (SSBR). Thus, the BER (SSBR) pathway plays a critical role in maintaining genomic integrity, and may help us to better understand the mechanisms of aging, tumor formation, and degenerative diseases. AP site (apurinic/apyrimidinic site) or damaged base excision, nucleotide insertion and ligation are included in the BER (SSBR) pathway, which are facilitated by different enzymes at each step. Most previous in vitro BER studies have used modified radiolabeled (32)P oligonucleotide substrates. Which is a very conventional method for in vitro BER assay. However, the use of radioactive isotope material was limited in various laboratories which are unable to handle radioactive hazard. In this study, we developed a novel technique using real-time quantitative PCR (qPCR) to quantify BER activity in in vitro assays. Single strand breaks, DNA ligase activity, and glycosylase activity were detected to establish the feasibility and advantages of this qPCR technique for in vitro BER profiling.

Codependent and independent effects of nitric oxide-mediated suppression of PhoPQ and Salmonella pathogenicity island 2 on intracellular Salmonella enterica serovar typhimurium survival

Here we show that the Salmonella enterica serovar Typhimurium PhoQ sensor kinase lessens the cytotoxicity of reactive nitrogen species (RNS) generated by inducible nitric oxide synthase (iNOS) in the innate response of mononuclear phagocytic cells. This observation is consistent with the expression patterns of PhoP-activated genes during moderate nitrosative stress in the innate host response. In contrast, RNS synthesized during high-NO fluxes of gamma interferon (IFN-gamma)-activated macrophages repress PhoP-activated lpxO, pagP, and phoP gene transcription. Because PhoP-regulated Salmonella pathogenicity island 2 (SPI2) genes are also repressed by high-order RNS (39), we investigated whether the NO-mediated inhibition of PhoPQ underlies the repression of SPI2. Our studies indicate that a third of the expression of the SPI2 spiC gene recorded in nonactivated macrophages depends on PhoQ. Transcription of spiC is repressed in IFN-gamma-primed macrophages in an iNOS-dependent manner, irrespective of the phoQ status of the bacteria. Transcription of spiC is restored in IFN-gamma-treated, iNOS-deficient macrophages to levels sustained by a phoQ mutant in nonactivated phagocytes, suggesting that most NO-dependent repression of spiC is due to the inhibition of PhoPQ-independent targets. Comparison of the intracellular fitness of spiC, phoQ, and spiC phoQ mutants revealed that PhoPQ and SPI2 have codependent and independent effects on S. Typhimurium survival during innate nitrosative stress. However, the intracellular survival of most S. Typhimurium bacteria is conferred by the PhoPQ two-component regulator, and the SPI2 type III secretion system is repressed by high-order RNS of IFN-gamma-activated macrophages.